1. Field of the Invention
Embodiments of the present invention relate generally to lithography, and more particularly to improving alignment targets in lithographic patterning processes, such as a double patterning process.
2. Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of resist material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The manufacture of an IC chip involves the fabrication of many layers. Within each layer, multiple or double patterning may be used to generate patterns with a smaller critical dimension than that achievable using a traditional patterning process. There are a number of different methods of achieving double patterning. The first of these is known as lithographic-etch-lithography-etch (LELE) in which a first pattern is exposed and etched. A second pattern, with features located in the spaces between the features of the first pattern, is then exposed. Next the features of the second pattern are etched into the substrate. Thus, a pattern of smaller dimensions than the minimum lithographic pitch can be generated. Another similar double patterning technique is known as lithography-freeze-lithography-etch (LFLE). A pattern is exposed in the resist, which is then “frozen”, usually by chemical fixation. A second pattern can then also be exposed in a freshly deposited resist layer and both patterns are then etched into the substrate. Another double patterning method is known as the spacer method. In the spacer method a sacrificial template is put down and spacers placed either side, and adjacent to, the sacrificial template. The template is then removed and the resulting pattern etched into the substrate.
Using LELE or LFLE, there exists the potential for error in the placement of features during the second lithography step with respect to the features from the first lithography step. However, in order to maximize the benefits of double patterning, it is imperative that the first and second lithography steps be aligned with little or no alignment error.